Apple talked a lot about updating iWork for free with the new Mavericks, I bought iWork '08 it was installed on my machine and updated with no issue.
A few months back I upgrade my MBP to a SSD, and did a clean install, I put iWork 08 on expecting it to be updated, but this hasn't happened.
Has this upgrade no longer available?
iWork '08 was released for OS 10.4 originally from 2007. Whilst it would have free upgrades within its cycle e.g. 8.1, .2, .3 effectively this does not mean you get a free upgrade to a significantly new version 3 years later e.g. iWork 09.
In your case, its likely there was a temporary offer for a free upgrade initially to encourage customers to port to the Apple Store (for software download) rather than traditional disc installation. After a period Apple withdraw software these upgrades.
In terms of your re-install, have you signed in with the same Apple Store account used previously? If you haven't, it wont recognise your previous status and upgrades may be un-available. This is true of the 10.8 OS update - if it is on your Apple Store account as installed previously, you can download it again, if you come into the Store clean, you wont even see it as a download.
You can/haveto download Pages/Numbers/Keynote (now separate apps, no longer available as a set called iWork) from the App Store. Wether you have to pay for them or they're free isn't depending on your operating system but on the age of your mac.
